1. Field of the Invention
The present invention relates to information communication systems. More particularly, the present invention relates to a system and method for mitigating noise associated with information communication.
2. Background Information
A conventional time domain reflectometer (TDR) operates by abruptly stimulating an object under test, and subsequently, over time, recording the elicited responses. Typically, the subject object is a medium designed for the propagation of energy from point to point in some form, such as, for example, sound, light or electricity. One type of TDR can analyze the electrical propagation characteristics of extended lengths of twisted pairs of wires, as are commonly used to transport telecommunications signals. Such a TDR is designed to interface with twisted pair wire lines that are deployed as a subscriber loop plant (the collective set of the pair of leads over which each subscriber has traditionally been provided fixed location telephone service) at either the service provider's, or subscriber's, end.
In operation, such a TDR emits a probe signal of an appropriate kind for the subject telecommunications medium. Non-uniformities in the telecommunications medium exist, and probe signal energy is lost progressively as a function of distance traveled in the medium, due to the unavoidable dissipative characteristics of the medium material. Any non-uniformity encountered as the probe signal stimulus propagates along the medium results in at least a partial reversal of probe signal energy flow, thus producing a return, or echo, signal that propagates back toward the stimulus source. These echoes, or reflections, are essentially returning signatures of aberrations in an otherwise continuous and uniform telecommunications transmission medium. The TDR records echoes, if any, from the moment of probe signal emission, as a function of time as the echoes arrive back at the source. Recording these echoes with respect to time provides a signature of the medium that reveals non-uniformities created either intentionally, or by accident, along the propagation path. In addition, the distance from the observation point to any particular aberration can be imputed from the corresponding return delay time relative to the probe signal stimulus, if the propagation speed for the medium is known.
This type of measurement has been unnecessary for subscriber loops until the recent deployment of wide-bandwidth digital services over wire line facilities originally intended only for narrow-bandwidth voice and modem signal transport. Now, defects that were previously inconsequential for the purposes of analog voice service can impair or even inhibit operation of, for example, xDSL (where “xDSL” refers to any variant of the Digital Subscriber Loop (DSL) technologies for transmitting high-bandwidth information over twisted-pair (i.e., copper wire) wire lines). A TDR provides a convenient means of identifying the type of, and distance to, a particular discontinuity or defect.
xDSL services can be offered over loop lengths as long as 18,000 feet from the telephone company office, without a repeater. At these lengths, energy losses at the frequencies necessary for xDSL operation can be so large that only approximately 1 part in 10,000 of the energy provided by the service provider ultimately appears at the subscriber end of the loop. It follows, then, that a defect near the subscriber, or far end of the loop, that will produce a reflection of a similarly attenuated incident TDR probe signal, can only be identified after traveling back to the TDR, again suffering the same ratio of energy reduction. Thus, a TDR must be able to identify energy as a probe signal echo that may be only 1 part in 100,000,000 as big as the issued probe signal. Since the probe signal cannot be made arbitrarily large, for both practical and regulatory reasons, much of the small signal recovery burden falls on the TDR receiver.
Receiver sensitivity is fundamentally limited by noise (i.e., unwanted electrical or electromagnetic energy that degrades the quality of signals and data) that can be created not only externally, but also within the receiver itself. Ideally, the contribution by the receiver, or the other portions of the apparatus containing the receiver, should be zero. For TDR, for example, since for practical purposes the state of the loop being probed is unchanging, or stationary, the probe signal can be sent numerous times, as long as enough time elapses between trials to allow all of the reverberating energy to dissipate. For incoherent noise (i.e., noise that is unrelated in periodicity to the receiver process or probe signal), a common mitigation method involves averaging the results from N trials which reduces the apparent noise energy by N. Unfortunately, this same process is not effective in mitigating coherent noise (i.e., noise that is related in periodicity to the receiver process or probe signal).
Conventional solutions for mitigating induced receiver noise can involve identifying the offending noise sources individually, and addressing, in turn, each of the dominant coupling mechanisms to the receiver. For example, if a switching power converter circuit is a noise producer, the receiver can be affected by conducted interference created by the switching operation itself or a poor transient response to a periodically changing load. Mitigation of the effects of this noise can involve, for example, adding filtering to further isolate the receiver from the vagaries of the switching power converter, correcting the transient response, or altering the load characteristics imposed on the switching power converter. Each of these mitigation measures can require design changes that can increase circuit board design complexity, and which can result in cumbersome and costly circuit board alterations. Furthermore, radiated electromagnetic fields from a switching power converter can play a role in disturbing a proximal receiver. This coupling mechanism can be mitigated by increasing the physical distance between the noise producer and the victim circuitry. If increasing physical distance is impractical as a result of, for example, circuit board size constraints, shielding can be added around specific components or entire blocks of circuit components of either or both of the involved circuits. Again, circuit board and mechanical design complexity increases with attendant size and cost penalties.